At least four people were killed in three attacks by suicide bombers in Nigeria's northeastern city of Maiduguri, police and witnesses said Wednesday.

The first attack late Tuesday involved a male bomber who was shot dead by a military sniper after he was seen moving toward a security checkpoint, said Damian Chukwu, police commissioner of Borno state.

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Then two teenage female suicide bombers attacked early Wednesday.

The first was shot dead by soldiers and the second blew herself up after civilian self-defense fighters stopped her from moving toward a mosque, Chukwu said. The bomber and one of the fighters were killed.

Maiduguri is the birthplace of the Boko Haram Islamic insurgency which has killed more than 20,000 people since 2009. The group continues to carry out deadly attacks despite the recent claim by Nigeria's president that it had been crushed.

On Monday, suspected Boko Haram extremists killed three people and kidnapped seven women in an attack on Ndagu village, located in the Askira Uba local government area of Borno state, Chukwu said.

"Most of the villagers fled but their houses were completely razed down by fire," he said.

The number of those kidnapped could be higher as some people remain unaccounted for, said resident Ibrahim Askira.

"As I speak with you now, many people who fled to the bush have not returned," he said.